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REGULATORY ASSETS AND LIABILITIES (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Jan. 01, 2017
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]        
Regulatory Assets, Current $ 54,420 $ 88,180    
Regulatory Assets, Noncurrent 437,479 405,189    
Regulatory Liability, Current (48,394) (9,438)    
Regulatory Liability, Noncurrent (520,866) (519,421)    
Net regulatory assets and liabilities, current 6,026 78,742    
Net regulatory assets and liabilities, noncurrent (83,387) (114,232)    
Net Regulatory Assets (77,361) (35,490)    
Amortization of Rate Deferral $ 1,700 $ 1,000 $ 3,800  
Federal income tax rate changes [Member]        
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]        
Regulatory Asset, Amortization Period See Note 13 See Note 13    
Regulatory Liability, Current $ (30,934) $ 0    
Regulatory Liability, Noncurrent (520,866) (519,421)    
Regulatory Liabilities $ (551,800) $ (519,421)    
Weather normalization [Member]        
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]        
Regulatory Asset, Amortization Period 1 year 1 year    
Regulatory Assets, Current   $ 17,461    
Regulatory Assets, Noncurrent   0    
Regulatory Assets   $ 17,461    
Regulatory Liability, Current $ (3,792)      
Regulatory Liability, Noncurrent 0      
Regulatory Liabilities $ (3,792)      
Over-recovered purchased-gas costs [Member]        
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]        
Regulatory Asset, Amortization Period 1 year 1 year    
Regulatory Liability, Current $ (13,668) $ (9,434)    
Regulatory Liability, Noncurrent 0 0    
Regulatory Liabilities (13,668) $ (9,434)    
Ad valorem tax [Member]        
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]        
Regulatory Asset, Amortization Period   1 year    
Regulatory Liability, Current   $ (4)    
Regulatory Liability, Noncurrent   0    
Regulatory Liabilities   (4)    
Total regulated liabilities [Member]        
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]        
Regulatory Liability, Current (48,394) (9,438)    
Regulatory Liability, Noncurrent (520,866) (519,421)    
Regulatory Liabilities $ (569,260) $ (528,859)    
Under-recovered purchased-gas costs [Member]        
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]        
Regulatory Asset, Amortization Period 1 year 1 year    
Regulatory Assets, Current $ 25,083 $ 41,238    
Regulatory Assets, Noncurrent 0 0    
Regulatory Assets $ 25,083 $ 41,238    
Pension and postretirement benefit costs [Member]        
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]        
Regulatory Asset, Amortization Period See Note 12 See Note 12    
Regulatory Assets, Current $ 23,384 $ 25,156    
Regulatory Assets, Noncurrent 421,726 387,582    
Regulatory Assets $ 445,110 $ 412,738    
Reacquired debt costs [Member]        
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]        
Regulatory Asset, Amortization Period 9 years 10 years    
Regulatory Assets, Current $ 812 $ 812    
Regulatory Assets, Noncurrent 6,487 7,298    
Regulatory Assets $ 7,299 $ 8,110    
MGP Costs [Member]        
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]        
Regulatory Asset, Amortization Period 15 years 15 years    
Regulatory Assets, Current $ 0 $ 0    
Regulatory Assets, Noncurrent 7,724 6,104   $ 5,900
Regulatory Assets $ 7,724 $ 6,104    
Ad valorem tax [Member]        
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]        
Regulatory Asset, Amortization Period 1 year      
Regulatory Assets, Current $ 1,070      
Regulatory Assets, Noncurrent 0      
Regulatory Assets $ 1,070      
Other regulatory assets [Member]        
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]        
Regulatory Asset, Amortization Period 1 to 20 years 1 to 21 years    
Regulatory Assets, Current $ 4,071 $ 3,513    
Regulatory Assets, Noncurrent 1,542 4,205    
Regulatory Assets 5,613 7,718    
Total regulatory assets, net of amortization [Member]        
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]        
Regulatory Assets, Current 54,420 88,180    
Regulatory Assets, Noncurrent 437,479 405,189    
Regulatory Assets $ 491,899 $ 493,369